Fifteen intact female monkeys were placed into three groups of five, receiving either an 18, 32 or 16 MG subdermally placed capsule of levonorgestrel (LNG) for groups I through III respectively on treatment day 0. Blood collections occurred as follows one pre treatment, daily from 1 through 7, once per week for 9 months for groups I and II, and 6 months for group III, then biweekly for 30, 17 and 16 months for groups I through III respectively. To study the release of LNG from the subdermal capsules into the circulatory system, levels of serum LNG were measured by radioimmunoassay. Serum analyzed for LNG showed levels dropping from 7.5 Ng/ml (sem 1.0) on day 1 to 0.43 Ng/ml (sem 0.25) On day 1127, 14.9 Ng/ml (sem 3.8) On day 1 to 1.39 Ng/ml on day 805 (n=1) and 10.9 Ng/ml (sem 1.6) On day 1 to 0.61 Ng/ml on day 760 (n=1) for groups I through III respectively. All animals in group I have remained on a biweekly blood collection schedule with blood collections ending on day 805 and 760 for groups II and III respectively. Implants were removed from four animals each on groups II and III respectively; six due to declining levels of LNG and two due to declining health status. The removed fragments were sent to an outside investigator for LNG analysis and degree of degradation of the capsule during insertion. Attempts at locating and removing the implants for the remaining animals from both groups II and III were unsuccessful.